Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

TOFD has been used for some time for general weld inspections. It has
proven to be a rapid and easily deployable method with an excellent capacity
for sizing. One of the inherent strengths of TOFD for detection and sizing
purposes is its relative indifference to the orientation of defects because of its
primary use of diffracted versus reflected energy.
The TOFD technique utilizes two transducers: a transmitter transducer floods
the inspected region with sound in the forward direction; on the opposite side
of the weld, a receiver transducer is positioned to receive diffracted and
reflected energy from the back wall or from anomalies present in the region.
Common pulse-echo techniques can be misdirected by the shape of the
region, resulting in imprecise measurement and assessment.